In all radio receivers, the first amplifier after the antenna when entering the receiver should be of especially low noise type, because the signal level at the input of this amplifier is low, and the additional noise caused by the amplifier is further amplified in all the following amplifier stages. Therefore, the low noise first amplifier is generally referred to by the acronym LNA. In base stations of mobile communication networks, the highest permitted noise figure specified for the front stage of a receiver including, in addition to a LNA, an antenna filter among other things, is typically 1.8 dB. This limit must be adhered to, even if the front stage included a switching arrangement for bypassing the LNA, which arrangement causes additional noise. The by-pass of the LNA is necessary for some measurements related to the maintenance of the base station and in a fault condition of the LNA.
FIG. 1 presents as block diagram a part of a typical radio receiver on the side of the antenna and a known way of arranging the by-pass of the low noise amplifier. The figure shows an antenna ANT and from it forward in succession on the signal path of the receiver an antenna filter 110, a first switch SW1, an amplifier unit 130 containing the LNA, a second switch SW2, a passband filter FIL and a mixer MIX. The antenna filter 110 is of the band-pass type in its entirety; physically it may contain a band-pass part and a low-pass part in succession. Switches SW1 and SW2 are change-over switches, and they have shared control C. When the switches are in position 1, the output signal of the antenna filter is directed through switch SW1 to the input of the LNA, and the output signal of the LNA through switch SW2 forward towards the mixer. When the switches are in position 2, the output signal of the antenna filter 110 is directed through switch SW1 to the by-pass path 140 and through switch SW2 forward towards the mixer. Thus the LNA is passed.
FIG. 2 presents as block diagram a front stage of a radio receiver, in which the amplifier unit contains two parallel amplifier branches, and a known way to arrange the by-pass of such an amplifier unit. By forming the amplifier unit 230 of parallel branches instead of a single LNA, the impedance matching of the amplifier unit towards the antenna filter is facilitated. In addition, a wider dynamic and linear area and a better stability are achieved. Because of the parallel amplifier branches, the signal E1 coming from the antenna filter 210 is divided into two similar parts E11 and E12 in the divider 220. The phase of the first division signal E11 is changed 90 degrees in the first phase shifter and is then amplified in the first LNA. The second division signal E12 is amplified in the second LNA, and the signal phase is then changed by 90 degrees in the second phase shifter. The partial signals, again of the same phase, are summed in the combiner 250, the output signal of which continues towards the mixer of the receiver.
The by-pass arrangement of the amplifier unit 230 is similar to that of amplifier unit 130 in FIG. 1 At the output of the antenna filter 210, before the divider 220, there is the first change-over switch SW1, by which the input signal E1 can be directed either to the divider and through it to the amplifier unit 230 or the by-pass path 240. By the second change-over switch SW2, either the signal received from the combiner 250 or the signal coming from the by-pass path can be connected to the mixer. The change-over switches are controlled by a shared control signal C.
In the arrangements of FIGS. 1 and 2, the first switch SW1 and the second switch SW2 can be implemented by pin diodes, MMIC components (Microwave Monolithic Integrated Circuit), MEMS switches (microelectro-mechanical system) or relays. The attenuation caused by a single switch on the signal is of the order of 0.25 dB. The attenuation of the switch on the front side of the LNA increases the noise figure of the receiving end by the same amount. On the output side of the LNA the effect of the switch on the noise figure is not so substantial. Another drawback caused by the switches is that in the normal operation state of the front stage, the separation attenuation, or isolation, to the by-pass path of the LNA is finite and not infinite. This again deteriorates the noise figure of the front stage. The switches can be designed to have very high isolation, but then the losses caused by them increase in practice.
FIG. 3 presents a by-pass arrangement known from the application publication EP1381162A2, by which the above mentioned drawback caused by the switches is reduced. The front stage is like the one shown in FIG. 1 in that it similarly includes an antenna filter 310, an amplifier unit 330 containing one LNA, a by-pass path 340 and a change-over switch SW of the output side. The difference to FIG. 1 is that the first change-over switch SW1 has been left out, and instead of it the antenna filter 310 has been provided with two parallel outputs. The first output OUT1 is connected directly to the input of the amplifier unit, and the second output OUT2 is connected directly to the by-pass path 340. The output of the amplifier unit is connected to the first change terminal of the change-over switch SW, and the by-pass path to the second change terminal of the change-over switch SW. By the control signal C it is determined which change terminal becomes connected to the output terminal of the change-over switch SW.
The antenna filter 310 is of the resonator type. Its parallel outputs have been implemented so that there are two coupling elements in the cavity of the last resonator on the signal path. Each of these is connected to its own connector on the wall of the filter. The switching elements are located in the resonator cavity such that they give out signals of the same level if the load were the same. During the normal operation of the front stage, the transmission path from the second output OUT2 towards the by-pass path of the amplifier LNA has a high impedance, and then it does not load the feeding source, or the antenna.
The solution according to FIG. 3 can as well be applied in the case of FIG. 2 by replacing its first switch SW1 by parallel outputs of the antenna filter.